Silicon carbide (hereinafter abbreviated as SiC) is a chemically stable material having a large band gap, and has been studied and highly expected to be a more favorable material for various semiconductor devices which can be used at a high temperature or when subjected to radioactive rays, as compared with silicon. While the conventional silicon devices have an operation limit of up to about 150.degree. C., it has been confirmed that a prototype of an element or device formed of SiC, such as a pn junction diode or a MOSFET (field-effect transistor of a metal-oxide-semiconductor structure), can be operated at 400.degree. C. or higher. If the use of the SiC device at a high temperature is feasible, a robot or a computer including SiC devices can be used under severe, inaccessible environments, for example, in a nuclear reactor or in space.
In addition, the conventional silicon device requires cooling equipment for avoiding a temperature rise due to heat caused by a generator loss during operation of the device. This makes the whole semiconductor device large-sized due to the presence of a cooling fin or other cooling equipment. If SiC is employed to form the semiconductor device, such cooling equipment can be significantly small-sized and simplified. The thus small-sized semiconductor devices may provide many components of an automobile, for example, assuring significantly reduced fuel consumption, which has a great effect on environmental conservation. Thus, the semiconductor devices formed of SiC have been highly expected to yield advantageous effects when used in many applications.
A vertical MOSFET is considered as an important or advantageous type of device when making an attempt to use SiC in a power semiconductor device, since the vertical MOSFET, which is a voltage-driven device, allows parallel driving of a plurality of elements and simplification of a drive circuit. Further, the vertical MOSFET is a unipolar element, and thus enables high-speed switching. While it is difficult to diffuse impurities into SiC, unlike silicon, it is relatively easy to utilize epitaxial growth to produce the SiC device. Therefore, it is generally known to provide a trench MOSFET having a trench 5 as shown in FIG. 4. FIG. 4 is a cross sectional view showing a principal part of the trench MOSFET formed of SiC, which has been developed up to the present. In the figure, an n drift layer 2 having a lower impurity concentration than an n.sup.+ substrate 1, and a p-type p base layer 3 are epitaxially grown on the n.sup.+ substrate 1 to provide a SiC substrate, and a n.sup.+ source region 4 having a high impurity concentration is formed in a selected area of a surface layer of the SiC substrate. A trench 5 is formed in a part of the n.sup.+ source region 4, to extend from the surface of the source region 4 down to the n drift layer 2. The device further includes a gate electrode 7 which is disposed inside the trench 5, through a gate insulating film 6. Further, a source electrode 8 is formed in contact with both the surface of the n.sup.+ source region 4 and an exposed portion of the surface of the p base layer 3, and a drain electrode 9 is formed on the rear surface of the n.sup.+ substrate 1.
In operation of the MOSFET constructed as described above, when a positive voltage of not lower than a predetermined level is given to the gate electrode 7, while a voltage is applied between the drain electrode 9 and the source electrode 8, an inversion layer appears in the surface layer of the p base layer 3 adjacent to the gate electrode 7, and electrons flow from the source electrode 8 toward the drain electrode 9 through the inversion layer. The gate insulating film 6 used in this SiC device may be a silicon oxide film formed through thermal oxidation of SiC.
Where the electric field in the insulating film and that in the semiconductor are respectively represented by Ei and Es, and the dielectric constant of the insulating film and that of the semiconductor are respectively represented by .epsilon.i and .epsilon.s, the relationship indicated by the following equation: .epsilon.i.multidot.Ei=.epsilon.s.multidot.Es is established at the interface between the insulating film and the semiconductor. Accordingly, the following equation : Ei/Es=.epsilon.s/.epsilon.i is established. This value Ei/Es will be calculated in both cases where the semiconductor is formed of silicon and SiC, respectively, and the insulating film is a silicon oxide film having a dielectric constant .epsilon.i of 3.8. In the case of silicon having a dielectric constant .epsilon.s of 11.7, the value Ei/Es is equal to 3.1. In the case of SiC having a dielectric constant .epsilon.s of 10.0, the value Ei/Es is equal to 2.6. Namely, the electric field applied to the gate insulating film of the conventional device of FIG. 4 is far greater than that applied to semiconductor parts of the same device. FIG. 5 is a graph indicating the distribution of the electric field in a gate portion of the device, along line A--A of FIG. 4. In the graph of FIG. 5, the axis of ordinates indicates the strength of the electric field, and the axis of abscissas indicates the depth. It will be understood from this graph that the strength of the electric field Ei of the insulating film is about three times as high as that of the semiconductor Es.
Further, the maximum electric field Esmax in the semiconductor is equal to 2.times.10.sup.5 V/cm in the case of silicon, and is equal to 2.times.10.sup.6 V/cm in the case of SiC. Accordingly, the maximum electric field in the insulating film Eimax is equal to 6.times.10.sup.5 V/cm in the case of silicon, and is equal to 5.times.10.sup.6 V/cm in the case of SiC. Assuming that the dielectric breakdown voltage of the silicon oxide film is about 8.times.10.sup.6 V/cm, a large electric field that is close to the breakdown voltage is applied to the gate insulating film when the avalanche breakdown starts within the semiconductor formed of SiC.
A power device is normally required to withstand a predetermined current when avalanche current flows through the device. In the conventional SiC trench MOSFET, however, the avalanche breakdown starts at the trench of the gate portion of the device, and the amount of resistance to the avalanche is limited to a considerably small value, due to the dielectric breakdown of the gate insulating film.
In the light of the above problems, it is an object of the present invention to provide a SiC trench MOSFET which is free from dielectric breakdown of a gate insulating film, and provides a large amount of resistance to avalanche breakdown.